In the papermaking process, a pulper is the device employed to convert cellulose materials into a defibered water slurry capable of being pumped to subsequent operations. Various types of pulpers have been developed, originating with a simple, open rotor revolving in an open top cylindrical tank. By providing a stationary bladed element with which the rotor blades can cooperate, additional defibering capability can be developed, thereby improving overall efficiency. With an open rotor, there is a limit to the amount of power (which ultimately is a function of rotor diameter, number and height of rotor blades, and RPM) which can be accommodated without creating an excessive vortex. By providing a stationary bladed element essentially surrounding the rotor, additional power can be accommodated, without causing unnecessary circulation, thereby transforming this additional power into useful energy.
However, operating at a fixed rotor/stator clearance, there is ultimately a limit to the amount of defibering (and the amount of fibre treatment) which can be accomplished in this manner. By providing for axial movement of the rotor with respect to stator, clearance can be adjusted and, in fact, reduced to essentially zero to effect more positive fibre separation. Beyond, however, mere defibering, it is possible to actually refine (i.e., cut and fibrillate fibres) by applying suitable thrust between rotor and stator surfaces. By requiring the fibres to pass between the rotor and stator under substantial thrust, a significant refining action can be produced which has a remarkable effect and not only completely defibres otherwise difficult-to-defibre materials, but actually contriutes to refining of the fibres themselves such that the slurry is in a condition whereby it can be pumped easily to subsequent operations.
Such treatment has been found to be required in the processing of unconventional material such as leather, certain tough rejects, hemp, flax, rags, and other materials. Because of the characteristic stringy, tangled nature of such materials, reduction to pumpable form has traditionally been carried out in beaters which, though effective, incur high labor, maintenance and energy costs. When processed in conventional pulpers, such materials not only resist reduction but they are impossible to defibre inasmuch as the action between bladed rotor and stator (at fixed clearance) is insufficient to break down the mass into a suitably defibered condition. Refiners on the other hand, can refine such material only if it has been suitably pre-treated and thus reduced in size so that it can be accommodated by the refiners.
The purpose of the present invention is to literally combine these two normally independent actions into one unit so as to, in effect, refine in the pulper. This can be accomplished by constructing a pulper rotor/stator combination such that the rotor is capable of axial adjustment to provide required thrust in a manner similar to a conical refiner. Necessarily, sufficient horsepower must be connected to permit delivery of required force. The mass of unconventional material is subjected to a grinding action which reduces the mass to a condition in which it can enter the interface and be further defibered and refined. Such a method will not only permit for positive separation of fibres, but will actually enable physical transformation of the fibres so as to provide fibre pre-treatment.
Prior art pulpers have not been able to cope with the unconventional materials specified above because they would never be defibered enough to be pumped out of the container and are so tough intrinsically, so tangled and interwoven as to form into a mass that will not yield to anything short of substantial thrust between rotor and stator edges in the manner of shears. Such a mass cannot be pulled apart by the hydraulic shearing affect of a conventional pulper, anymore than the tangled mass could be cut with a pair of loose, dull scissor blades. It needs grinding and shearing to permit further treatment.